The present invention is directed to metal alloys for use in jewelry, especially for use in the fabrication of relatively high weight academic rings and corporate recognition jewelry products.
The customary jewelry alloys having a gold content between 41 and 75 weight percent are well suited for the production of jewelry. Alloy compositions such as these are resistant to corrosion, investment cast easily and have an esthetic yellow color.
In more recent times, because of the high price of gold, alloys have been developed for ring casting that contain lower gold weight percentages than the traditional 10K, 14K and 18K. Additions of indium and palladium in various ratios along with increased weight percentages of silver have been widely employed to increase corrosion resistance in dental formulations as well as jewelry alloys. Davitz U.S. Pat. No. 4,895,701 discloses a corrosion resistant gold-free dental compositions resembling 10K that contain about 18% copper, 25% palladium, 21% indium and the balance being silver. In this case the resulting yellow color is reportedly due to a reaction of copper and indium in the presence of palladium. Van Der Zel U.S. Pat. No. 4,992,297 teaches that combinations of palladium and indium can produce yellow colors through the formation of the brittle intermetallic compound B-PDIN and that in an alloy consisting of approximately equal ratios of palladium, indium and silver, the addition of gold up to 10% increases the hardness and has the adverse effect of reducing the yellow color.
More common are alloys for dental and jewelry applications that contain 15 to 40 weight percent gold along with lesser additions of palladium, indium, copper, zinc and, for the enhancement of specific metallurgical properties, trace elements. Several U.S. patents describe gold colored alloys for use in dentistry and jewelry with and without copper and zinc but compositions with less than 11% by weight gold contain indium. Generally, as the by weight percentage of gold decreases, the indium weight percent increases.
Since indium bearing alloys are often subject to irreversible chemical changes under melting conditions very similar to normal operating parameters, cost advantages of using a lower gold quality material for ring casting can be lost due to poor quality castings and excessive proportions of unusable scrap. Normally, the intrinsic value of indium is significantly higher than silver, but because of complications in chemical refining, indium is often ignored or lost into waste while silver, gold and platinum group metals are effectively recovered. Further, indium presents a number of worker health concerns when melted in poorly ventilated areas. What is desired is a low gold, corrosion resistant yellow alloy which avoids disadvantages described in the prior art.